目录：

1. 如何绘制
2. 如何响应点击 -事件系统
3. 如何重绘ReBuild

1. 如何绘制

将顶点数据填充到Mesh，再将Mesh设置到Mesh filter。

1. 基础组件：

   1. Mesh filter 网格
   2. Mesh Render 渲染

2. 核心概念

   1. Mesh-网格: 决定物体的形状

           var mesh = new Mesh();
           m_VertexHelper.AddVert(new Vector2(0, 0),color, new Vector2(0, 0.5f));
           m_VertexHelper.AddVert(new Vector2(0, 1),color, new Vector2(0, 1));
           m_VertexHelper.AddVert(new Vector2(1, 1),color, new Vector2(1, 1));
           m_VertexHelper.AddVert(new Vector2(1, 0),color, new Vector2(1, 0));
           m_VertexHelper.AddTriangle(0, 1, 2);
           m_VertexHelper.AddTriangle(3,0, 2);
           m_VertexHelper.FillMesh(mesh);
      

   2. Vertext-顶点: 组成Mesh的元素

           m_VertexHelper.AddVert(new Vector2(0, 0),color, new Vector2(0, 0.5f));
           m_VertexHelper.AddVert(new Vector2(0, 1),color, new Vector2(0, 1));
           m_VertexHelper.AddVert(new Vector2(1, 1),color, new Vector2(1, 1));
           m_VertexHelper.AddVert(new Vector2(1, 0),color, new Vector2(1, 0));
      

   3. Triangle-三角形: 决定顶点的顺序

            m_VertexHelper.AddTriangle(0, 1, 2);
           m_VertexHelper.AddTriangle(3,0, 2);
      

   4. UV-纹理坐标

           m_MeshRenderer.material.color = color;
           m_MeshRenderer.material.mainTexture = texture2D;
      

3. 与组件结合使用

   m_MeshFilter.mesh = mesh;
   
   m_MeshRenderer.material.color = color;
   m_MeshRenderer.material.mainTexture = texture2D;
   

2. 如何响应点击

2.1 获取点中的UI元素： GraphicRaycaster

在GraphicRaycaster中调用Raycast：

Perform a raycast into the screen and collect all graphics underneath it.

1. 获取当前canvas下的所有Graphic ：var foundGraphics = GraphicRegistry.GetGraphicsForCanvas(canvas);
2. 遍历Graphic，判断Graphic 的raycastTarget，是否可以响应点击
3. 判断触摸点是否在Graphic的范围内（推测RectangleContainsScreenPoint函数中使用射线检测）： RectTransformUtility.RectangleContainsScreenPoint(graphic.rectTransform, pointerPosition, eventCamera)

注意事项：不需要响应点击的Graphic 不勾选Raycast Target，提高性能

    private static void Raycast(Canvas canvas, Camera eventCamera, Vector2 pointerPosition, List<Graphic> results)
    {
        // Necessary for the event system
        var foundGraphics = GraphicRegistry.GetGraphicsForCanvas(canvas);
        for (int i = 0; i < foundGraphics.Count; ++i)
        {
            Graphic graphic = foundGraphics[i];

            // -1 means it hasn't been processed by the canvas, which means it isn't actually drawn
            if (graphic.depth == -1 || !graphic.raycastTarget)
                continue;

            if (!RectTransformUtility.RectangleContainsScreenPoint(graphic.rectTransform, pointerPosition, eventCamera))
                continue;

            if (graphic.Raycast(pointerPosition, eventCamera))
            {
                s_SortedGraphics.Add(graphic);
            }
        }
    ...
    }

2.2 事件触发过程

ExecuteEvents：Execute、EventFunction

Button Click触发过程.png

射线检测鼠标位置下的Graphic.png

事件触发-EventTrigger中的事件.png

3. 如何重绘

1. UI发生变动时，会将发生变动的元素加入渲染队列（CanvasUpdateRegistry）
2. 注册Canvas.WillRenderCanvases事件，UI发生变动，该事件触发。

在CanvasUpdateRegistry内部，需要关注的方法是PerformUpdate。当Canvas组件触发WillRenderCanvases事件时都会调用这个方法。这个事件每一帧都会执行一次。

3. 该事件从渲染队列中获取元素，分别调用ReBuild方法
4. ReBuild方法中完成网格、材质等更新

1.绘制过程.png

2.图形绘制调用图.png

3.PerformUpdate.png

参考资料

1. 源码地址：https://bitbucket.org/Unity-Technologies/ui/downloads/?tab=tags
2. UGUI源码深度剖析，视频教程地址：http://study.163.com/course/courseLearn.htm?courseId=1003903003#/learn/video?lessonId=1004650011&courseId=1003903003
3. http://www.cnblogs.com/chiguozi/p/6770468.html